Internal combustion engines can be classified into two major categories based on the heat addition portion of their respective thermodynamic cycles: “constant volume” and “constant pressure” heat addition engines (cycles). For example, see Gülen, S. C., 2013, “Constant Volume Combustion: The Ultimate Gas Turbine Cycle,” Gas Turbine World, November/December 2013, pp. 20-27. Either process is an idealized conceptualization of the actual fuel-air combustion that takes place inside the actual engine. In particular,                Constant volume heat addition is closely approximated by the combustion of a fuel-air mixture within the cylinders of a reciprocating or piston engine, e.g., a car or truck engine.        Constant pressure heat addition is closely approximated by the combustion of a fuel-air mixture inside the combustor of a gas turbine.        
Thermodynamic cycle analysis, whether using the idealized air-standard approach or “real fluid or gas” approach, demonstrates the superiority of constant volume heat addition or combustion process in terms of cycle thermal efficiency. See also, Gülen, S. C., 2010, “Gas Turbine with Constant Volume Heat Addition,” ESDA2010-24817, Proceedings of the ASME 2010 10th Biennial Conference on Engineering Systems Design and Analysis, Jul. 12-14, 2010, Istanbul, Turkey. A reason for that, in layman's terms, is that constant volume combustion is a confined chemical explosion, with simultaneous increase of temperature and pressure of the working fluid. Other things being equal, this leads to better thermal efficiency because part of the compression is achieved within the heat addition part of the cycle and, for the same amount of heat addition, leads to higher net cycle power output (less compression work).
In terms of practical applications, these advantages are inherent efficiencies of modern gas fired reciprocating engine gen-sets (e.g., Wärtsila's 18V50SG) that are nearly 50% efficient (compared to around 40% for modern heavy-duty industrial gas turbines or 45% for smaller aeroderivative units with high cycle pressure ratios).
Efforts to exploit constant volume combustion (CVC) in the context of gas turbines goes back to Holzwarth's explosion turbine in the early years of the 20th century. The intermittent nature of CVC combustion within a confined space (similar to the “explosion” of fuel in an engine cylinder in the space between the piston and cylinder head), is contrary to the continuous flow nature of turbine combustion generally, characterized by combustion in an “open system” as part of a steady-state steady-flow (SSSF) process. As a result, CVC has dropped off the evolutionary trajectory of gas turbine technology for land-based electric power generation.
Similar ideas have persisted with respect to aircraft propulsion. A specific version of quasi CVC in this context is known as “pulse detonation combustion”. The engine comprising the pulse detonation combustor is known as a “pulse detonation engine” (PDE), discussed in Gülen, supra. As the name suggests, the concept involves creation of a detonation wave within a semi-closed tube filled with a fuel-air mixture. The resulting wave simultaneously compresses and heats the mixture, which is discharged into an axial turbine. The same dichotomy mentioned above, namely a “steady flow open system” versus an “intermittent flow closed system,” results in mechanical design difficulties, which so far have prevented the transition of PDE or similar CVC concepts into viable commercial products.
While CVC has not been commercialized as an integral part of a gas turbine engine (or cycle), the two types of internal combustion processes and respective engines (piston and turbine) have been tried in a “compound” system with some success. Early examples of “turbocompound” engines are Allison's V-1710-127(E27) and the Napier Nomad aircraft engine. Development of Allison's engine began in 1943 to power the Bell P-63H airplane. Both the engine and the airplane were built, but they were never flown due to the end of the war and the introduction of jet engines. The engine was rated at 3090 bhp at 3200 rpm and 28,000 feet, with a manifold pressure of 100 inHgA (˜50 psia) and an impressive specific fuel consumption rate of 0.365 lb/bhp-hr.
Developed in the UK in 1950s, the Nomad comprised a 12-cylinder two-stroke diesel engine in two six-cylinder blocks, also serving as a gas generator for a gas turbine. Both the diesel engine and the gas turbine contributed shaft power to a propeller, via a complicated gear arrangement. Nomad was considered the most efficient internal combustion engine flown, with less than 0.35 lb/bhp-hr in flight delivering about 3,000 bhp.
Despite the fuel efficiency offered by turbocompound engines, the aircraft industry bypassed them in favor of rapidly emerging gas turbines. Many factors played into the shift in the aviation industry, including weight/thrust ratio, cost, reliability, operational speed, fuel costs, etc. The technology is applied to land-based propulsion. For example, turbocompound diesel engines power some Scania (formerly Saab) trucks.
A turbocompound engine should not be confused with a “turbocharged” engine. A turbocharger is fundamentally different. In a turbocharged engine, exhaust gas coupled through a turbine operates a compressor unit to compress combustion air before it enters the engine cylinders. The turbocharger is merely an accessory for the piston engine, used to increase the working fluid mass for increased shaft power from the piston engine. In the turbocompound arrangement, a gas turbine is an “equal partner” with its reciprocating/piston counterpart. Both the piston engine and the turbine contribute to total shaft power generation.
A turbocompound gas turbine combined cycle concept has been proposed by Tsuji, T., 2005, “Cycle Optimization and High Performance Analysis of Engine-Gas Turbine Combined Cycles,” GT2005-68352, ASME Turbo Expo 2005, Reno-Tahoe, Nev., USA, Jun. 6-9, 2005; and Tsuji, T., 2007, “Performance Analysis on Gas Engine-Gas Turbine Combined Cycle Integrated with Regenerative Gas Turbine,” GT2007-27198, ASME Turbo Expo 2007, Montreal, Canada, May 14-17, 2007. The concept is named alternatively as an Engine Turbo-Compound System (ETCS), or as an Engine Reheat Gas Turbine (ERGT)
The ETCS/ERGT concept involves gas turbine exhaust gas heat recovery via a heat recovery steam generator (HRSG) for additional power generation in a steam turbine (ST). As such, it is a combined cycle system. In particular, ETCS is a true turbocompound concept where the two distinct internal combustion engines are separate entities in their own right as shown in FIG. 1 (labeled prior art).
Briefly, the ETCS system shown in FIG. 1 comprises a modified gas turbine gen-set GT and a modified gas engine gen-set GE. As to the modified gas turbine: Suction air is supplied at inlet 1 to compressor C. A portion of the compressed air from the discharge 2 of compressor C is sent to the gas engine GE after first being cooled in a heat exchanger (HX). Gas engine GE is a gas-fired reciprocating (piston-cylinder) engine in the gen-set with generator GEN2. Exhaust gas from gas engine GE is piped at exhaust 4 back to the gen-set including gas turbine GT, namely through a combustor (CB) inlet after having been mixed with compressed air bypassing the GE.
The modified gas engine GE does not have a turbocharger as an inlet accessory, but part of the air from outlet 2 of compressor C is coupled to the inlet 3 of gas engine GE, providing an intake charging function. Another result of intake charging to gas engine GE is that the exhaust gas at 4, coupled to the inlet 5 of turbine T in the gas turbine gen-set GT, is at a pressure high enough to satisfy the turbine (T) requirements.
The Tsuji ETCS is an integrated system, and might possibly be produced by combining and modifying “off-the-shelf” GT and a GE units to include various additional piping, heat exchangers, generators and other elements so as to interact as desired. The Tsuji ERGT is more an explanation of a conceptual model to address the thermodynamics underlying the ETCS system. These disclosures are characterized by double combustion as shown in FIG. 1, first in the gas engine cylinders (note fuel flow f2) and then in the combustor CB of the gas turbine (fuel flow f1), namely a “reheat” concept implicit in the ETCS. The reported ETCS performance is summarized in the following Table 1, which compares expected performance of ETCS configurations of substantially different sizes:
TABLE 1ETCS PERFORMANCE (Tsuji, T.)ETCS (1)ETCS (2)Type and Number6 MW-Class GT × 1150 MW-Class GT × lof Gas Turbine and(TIT 1150° C.)(TIT 1350° C.)Gas EngineGas Engine × 2Gas Engine × 2(900° C. Exhaust)(900° C. Exhaust)Power OutputGas Turbine6,700 kW160,400 kWGas Engine11,500 kW 200,100 kWSteam Turbine4,000 kW 99,100 kWETCS22,200 kW 459,600 kWThermal Efficiency (Gross, LHV Base)ETCS49.8% LHV 56.7% LHV